A stimulated Raman scattering (SRS) detection apparatus that is one of detection apparatuses utilizing a Raman scattering principle has been proposed in “Principle confirmation of stimulated Raman scattering microscopy” by Fumihiro Dake et al. and “Label-free biomedical imaging with high sensitivity by stimulated Raman scattering microscopy” by Chiristian W. Freudiger et al. The principle of the SRS detection apparatus is as follows.
When two light pulses whose light frequencies are mutually different are focused onto a sample, a coincidence of a difference between the light frequencies of the two light pulses with a molecular vibration frequency causes a phenomenon of stimulated Raman scattering at a light-focused point, the stimulated Raman scattering modulating intensity of the light pulse. Detection of an intensity-modulated light component (detection light) from the light pulse intensity-modulated by the stimulated Raman scattering and emerged from the sample enables molecular vibration imaging in which vibration information of molecules of the sample is reflected.
However, the SRS detection apparatus requires a light source generating extremely low noise close to a shot noise limit, which provides a restriction to a laser light source. Thus, the SRS detection apparatus needs a light source using a solid laser or an optical parametric oscillator which requires frequent extensive maintenance.
On the other hand, a stable small laser source such as a fiber laser generates a large intensity noise component in its laser output because the laser output is small. Thus, deterioration of an S/N (signal-to-noise) ratio of the laser source due to an influence of the intensity noise component becomes a problem when such a small laser source is used for the SRS detection apparatus.